Conventionally, planar targets are used for deposition by a sputtering method. However, in recent years, cylindrical targets, such as those mentioned in Patent Documents 1 to 3, have been used because of their excellent usability. For instance, an aluminum target used as the wiring material for liquid crystal displays (LCDs) or the like is usually cylindrical in shape with a length of 1,000 to 3,000 mm. A cylindrical target is used in a deposition method that involves sputtering while rotating the target. A plurality of cylindrical targets is usually vertically installed as a set in a sputtering apparatus.
If such kind of a cylindrical target has distortion, abnormal discharge is likely to occur during sputtering, or the thickness of a deposited film will become uneven. In general, most cylindrical targets are long, and hence distortion, particularly in the longitudinal direction of the target, is problematic. If any distortion occurs in the longitudinal direction of even one of the plurality of cylindrical targets, malfunction of the cylindrical target could occur due to the contact of the cylindrical target with a magnet, which has been inserted into the cylindrical target, or abnormal discharge could be caused by local approach or contact between the cylindrical target and its peripheral member, such as a shield, attached to an outer part of the cylindrical target. Furthermore, the cylindrical target is used while being rotated as mentioned above. Because of this, if the cylindrical target has distortion in the longitudinal direction, the distance between the cylindrical target and a substrate on which the film is deposited becomes uneven. This would make the thickness of the deposited film uneven or could lead to the exertion of a load on a connection portion that lies between the cylindrical target and the sputtering apparatus, causing damage to or failure of the sputtering apparatus.
When a large-sized cylindrical target having a length exceeding 1 m is used in a sputtering apparatus, normally, not one, but three to twenty cylindrical targets are used as a set. However, the presence of distortion in even one target of the target set in the longitudinal direction would lead to the malfunction, abnormal discharge, or uneven deposition, as mentioned above. In practice, cylindrical targets all of which have no distortion in the longitudinal direction cannot be prepared as a set.